In the hydrogenation of a hydrocarbon oil in the presence of hydrogen for hydrogen addition, desulfurization, denitrogenation, decomposition or the like, a hydrogenating catalyst composed of a catalyst carrier of a porous inorganic oxide, such as alumina, titania or silica, and active metal components of Group VI of the Periodic Table of Mo or W and the Group VIII of same of Ni or Co, as carried on the carrier, is used. In the hydrogenation catalyst of this type, in general, the above-mentioned active metals as carried on the catalyst carrier are in the form of their oxides Since such metal oxides are inactive as they are, the catalyst with such metal oxide cannot be applied to hydrogenation. Therefore, the metal oxides are necessarily activated, by converting them into the corresponding metal sulfides. For this reason, after the above-mentioned catalyst has been placed in a device for hydrogenation of a hydrocarbon oil, an adequate pre-sulfurization treatment is indispensable, wherein the catalyst layer in the device is necessarily sulfurized by introducing a hydrocarbon oil containing a dissolved agent under heat. It is known that the active site of the thus-pretreated catalyst is formed on the surfaces of the resulting active metal sulfides so that the total number of active sites will increase with an increase in the exposed surface area of the active metal sulfides, yielding, a high catalyst activity. An increase in exposed surface area of the active metal sulfides may be attained by enhanced dispersion of the active metal sulfides as carried on the catalyst carrier or by minimization of the crystal size of the respective active metal sulfides. Methods of preparing a catalyst having fine crystals of active metal sulfides finely dispersed and carried on a carrier have been disclosed. For instance, Japanese Patent Application Laid-Open Nos. 59-102442 and 59-69147 mention a series of methods of preparing a catalyst in which a catalyst carrier such as an alumina is dipped in an aqueous solution of active metals containing a carboxylic acid, such as citric acid or malic acid, as a complexing agent for active metals and thereafter it is fired. EP 0181035(A2) mentions a method of preparing a catalyst in which an organic compound having a nitrogen-containing ligand (e.g., amino group, cyano group) such as nitriloacetic acid, ethylenediaminetetraacetic acid or diethylenetriamine is used as a complexing agent and is added to an aqueous solution of active metals, a catalyst carrier such as an alumina or silica is dipped in the resulting aqueous solution of active metals, and the catalyst composed of active metals carried on the catalyst carrier is then dried at a temperature of not higher than 200.degree. C. without firing.
In accordance with the method of adding a carboxylic acid as a complexing agent followed by firing, the carboxylic acid added is effective for elevating the stability of the active metal-dipping solution as it acts as a complexing agent for active metals and additionally the acid is also effective for inhibiting coagulation of active metals. In the method, however, the active metals will finally coagulate because of the final firing step so that the finally obtained catalyst by the method cannot have a sufficiently improved catalyst activity. The method is defective in this point. On the other hand, in accordance with the method as disclosed in EP 0181035(A2), since the active metal ions such as Mo or Ni ions are firmly coordinated with the nitrogen-containing compound, such ions are carried on the carrier in the form of a highly dispersed condition. In addition, since the catalyst with the carrier is not calcined but is merely dried at a low temperature of not higher than 200.degree. C., the dispersion of the active metals is finally maintained as it is. Further, since the active metal ions are directly formed into sulfides thereof without forming oxides thereof by pre-sulfurization, the finally obtained hydrogenation catalyst may have an extremely highly dispersed state. For these reasons, the catalyst prepared by the method has a higher activity than any other conventional catalysts, but it cannot still does not have a high enough hydrogenation and desulfurization activity enough to meet the recent demand of reducing the sulfur content in a light oil (precisely, to 0.05% by weight or less as the sulfur content in a light oil) for solving the problem resulting from the current legal controls on gaseous wastes. Additionally, since the catalyst contains a nitrogen-containing organic compound such as nitriloacetic acid or the like, there is a problem that the organic compound would be decomposed in the pre-sulfurization step to generate a toxic gas such as hydrogen cyanide.